The fabrication of conventional storage media including non-volatile memory devices, faces many challenges as storage density increases and individual memory storage cell size decreases. Magnetic random access memory devices have several attractive features. Unlike conventional random access memory chip technologies, in MRAM data is not stored as electric charge or current flows, but by magnetic storage elements. Moreover, unlike dynamic random access memory, MRAM devices are all non-volatile and do not require refreshing to preserve the memory state of a cell.
In a simple version, the storage elements are formed from two ferromagnetic plates, each of which can hold a magnetic field, separated by a thin insulating layer. One of the two plates is a permanent magnet set to a particular polarity, the other's field can be changed to match that of an external field to store memory. This configuration is known as a spin valve and is the simplest structure for a MRAM bit. A memory device is built from a grid of such “cells”.
The simplest method of reading is accomplished by measuring the electrical resistance of the cell. A particular cell is typically selected by powering an associated transistor that switches electric current from a supply line through the cell to ground. Due to the magnetic tunnel effect the electrical resistance of the cell changes due to the orientation of the fields in the two plates. By measuring the resulting current, the resistance inside any particular cell can be determined, and from this the polarity of the writable plate. Typically if the two plates have the same polarity this is considered to mean “1”, while if the two plates are of opposite polarity the resistance will be higher and this means “0”.
A recent variant of MRAM, spin-transfer torque random-access memory, or STT-RAM, has the advantages of lower power consumption and better scalability over conventional magnetoresistive random access memory (MRAM), which uses magnetic fields to flip the active elements. Spin-transfer torque is an effect in which the orientation of a magnetic layer in a magnetic tunnel junction or spin valve can be modified using a spin-polarized current. The effects are usually most evident in nanometer scale devices. Accordingly, as device sizes of non-volatile memories scale to sub 100 nm dimensions, the use of STT-MRAM becomes more attractive.
Patterning of MRAM devices such as STT-MRAM may take place by defining a patterned mask that is formed on top of a stack of layers that contains at least two magnetic layers separated by an insulating layer. The patterned mask typically contains isolated mask features that expose regions of the substrate that lie between the mask features, which exposed regions are subsequently etched away. The remaining regions protected by the patterned mask features are unetched and define MRAM cells that are physically isolated from one another after etching. This patterning process entails many issues including the need to define individual memory cells by subtractive etching. For one, a typical MRAM cell does not have the shape of a vertical pillar in which the sidewalls are perpendicular to the plane of the substrate. Moreover, MRAM cells typically comprise many different layers having many different materials which provide a challenge for etching the layers to define MRAM cells. In addition, after etching, the sides of various layers in an MRAM cell may be subject to etching attack and/or deposition of unwanted material.
In view of the above, it will be appreciated that there is a need to improve patterning technologies to create patterned MRAM devices.